1. Field of the Invention
The present invention relates to a switching power supply to stabilize the voltage of a DC output by controlling the switching of a switching transistor according to a voltage error of the DC output, and in more detail, to a switching power supply, in which, when the switching transistor conducts the intermittent switching, the maximum value of a current of the switching transistor is limited to a current value smaller than the current value determined by the output of an error detection circuit.
2. Description of the Related Art
There is an RCC type switching power supply as one of switching power supplies for use in a power supply section of a television receiver. However, in the RCC type switching power supply, when the electric power consumed on the secondary side is smaller than the electric power supplied by the primary side because the load is greatly lightened, the intermittent switching in which switching is stopped at a period of several mS to several 100 .mu.S, occurs. Accordingly, a disadvantage that unpleasant oscillation noise in the audio band is generated from the transformer, occurs.
A conventional technique to solve such a disadvantage is proposed in Japanese Patent Unexamined Publication No. Hei. 7-163143. That is, in this technique, a second oscillation stop command section to intermittently drive the switching circuit at a set period out of the audio range when a standby signal showing the stoppage of the supply of the electric power to the load is sent out, is provided. Accordingly, the switching circuit conducts an intermittent switching operation whose period is forced, according to the output of the second oscillation stop command section. Accordingly, because the period of the intermittence is the period out of the audio band, the oscillation noise generated from the transformer becomes also the frequency out of the audio band, thereby, the unpleasant oscillation noise is prevented from generating.
However, when the above structure is used, the following problems occur. That is, the period when the second oscillation stop command section intermittently drives the switching circuit, is set by a time constant circuit composed of a smoothing capacitor and a resistor for a time constant.
Accordingly, in order to put the period of the intermittent drive out of the audio band, it is necessary to reduce the time constant. On one hand, in order to reduce the time constant, there are two methods; one in which the capacity of the smoothing capacitor is reduced, and the other in which the resistance value is reduced. However, in the case where the capacity of the smoothing capacitor is reduced, when a predetermined current is supplied to the load, the capacity of the capacitor is insufficient, and a DC output having a lot of ripples is supplied to the load. On the other hand, in the case where the resistance value is reduced, because the current value flowing to the resistor is increased, a problem occurs that a loss is increased.